The present invention relates to an apparatus for measuring a total flux of a lamp by the use of an integrating photometer. The integrating photometer is in wide practical use today in the field of optical radiometry.
It is desirable in the development or quality control of incandescent lamps or various discharge lamp, to accurately measure the total flux value of a lamp.
In measuring the total flux of the lamp, an integrating photometer is generally used. The integrating photometer is a device which comprises an integrating sphere having a photometric window defined in a wall thereof, a spectrometer for measuring light transmitted through the window, a lamp holder for holding and lighting a lamp to be measured inside the integrating sphere, and a baffle for shielding that portion of light, emitted from the lamp when the lamp is turned on, which travels directly towards the window.
The entire interior wall surface of the integrating sphere is a white diffusion surface so as to permit the interior wall surface to exhibit a uniform illuminance when radiated by the lamp. The uniformed illuminance is caused by the effect of interreflection taking place inside the integrating sphere.
A total flux measuring apparatus currently in use measures the total flux of the lamp by comparing it with a known total flux value of a standard lamp.
When the lamp to be measured is turned on, the flux emitted from this lamp is interreflected inside the integrating sphere. The white diffusion surface of the inner wall exhibits a uniform illuminance. This uniform illuminance is detected by a photometer which is connected to the integrating sphere. This output is compared with the value which the total flux standard incandescent lamp exhibit.
Because the lamp is turned on inside the integrating sphere during the measurement, the integrating photometer can be used in a bright room. For this reason, the integrating photometer is utilized not only in total flux measurement, but also in spectral distribution measurement wherein a spectrometer is mounted on the photometric window. In addition, the integrating photometer is widely used in lamp manufacturing factories or in lamp development sections.
The integrating photometer, however, has a problem, namely, the lamp holder causes measurement errors by casting a shadow through direct or indirect light and by absorbing the light.
This problem is especially evident when using a straight tube fluorescent lamp as an illuminant to be measured. The lamp holder for a straight tube fluorescent lamp differs from a total flux standard incandescent lamp. This may cause a big error. In addition, discharge property of a low voltage mercury vapor lamp such as a fluorescent lamp is determined by the low temperature luminous part of the lamp. When this lamp is the illuminant to be measured, the heat and radiation emitted from the lamp cause an increase of temperature of the integrating sphere which is a closed space. Thus, it may be difficult to measure the total flux accurately.
An integrating sphere having diameter of 1.5 m is used. A cylindrical lamp holder having the same radius of the straight tube fluorescent lamp, to be measured is installed along the diameter of the integrating sphere and applied with the same light diffusing material BaSO.sub.4 as the interior wall of the integrating sphere. The lamp holder is also coated with BaSO.sub.4. The total flux of a 40 W straight tube type fluorescent lamp is measured.
When the diffuse reflectance of BaSO.sub.4 is 95%, the absorption of flux by the lamp holder is about 2 to 3% of total flux.
In measuring the total flux in practice, total flux of the illuminant to be measured is estimated by comparing the total flux standard incandescent lamp.
The lamp holder of the total flux standard incandescent lamp is within the integrating sphere. Absorption of the lamp holder depends on shape and spatial distribution of luminous intensity of the illuminant. In addition, the illuminant has a self-absorption of the flux.
These factors cause a 5% error in total flux measurement.
To correct the error, a supplement lamp is installed in the integrating sphere. The absorption of total flux standard incandescent lamp with lamp holder and luminant to be measured with lamp holder are measured respectively. This allows a measurement to be performed to correct the error.